1. Field of the Invention
The present invention relates to a ballast for a discharge lamp, and more particularly, an electronic ballast provided with a voltage converter which provides a DC power from a DC source voltage for operating the discharge lamp.
2. Description of the Prior Art
Japanese Patent No. 10-511220 discloses a ballast for a discharge lamp which includes a voltage converter providing a DC power from a DC source voltage for operating the discharge lamp, and a controller for controlling DC power in a feedback manner based upon an output condition of the voltage converter. The voltage converter includes an inductance and a switching element which is controlled by the controller to turn on and off in order to accumulate an energy in the inductance and allow the energy to release from the inductance for applying the resulting power to operate the discharge lamp. The controller is basically configured to give a varying OFF-period of the switching element for effecting a mode in which the switching transistor is turned on upon seeing a zero-current flowing through the inductance, thereby enhancing switching efficiency. The controller is also configured to give a forced OFF-end limit for the varying OFF-period so as to turn on the switching element forcibly after an elapse of a predetermined period even if the current from the inductance does not reduce to zero, whereby avoiding a condition where the OFF-period becomes too long due to a prolonged time by which the current from the inductance reduces to zero, for prevention of a switching frequency of the switching element becoming too low. Further, the controller is configured to turn off the switching element to terminate the ON-period only after the current being fed to the inductance increases to a predetermined high level. However, this may pose a problem that the switching element should fail to turn off when the current to the inductance does not increase to the predetermined high level, as seen in a case where the DC source voltage gives a high impedance. Further, if the switching element is turned off after a long period, i.e., the ON-period becomes considerably long, the switching frequency would goes too low into an audible range, which is not acceptable for the ballast.